Currently Apple only sell the Apple Watch with a strap, but they can't expect everyone to buy straps again each time they want to upgrade the watch, so it follows that future Apple Watches will be available to buy without a strap? This would mean a watch-only price would be knowable. I'm guessing it would cost more to buy watch + strap separately than together. Also, while we don't know Apple's plans for a hardware refresh cycle, on the basis they make changes to the physical hardware, it is quite likely at some point the strap connection size will change (as the watch gets smaller). So at some point, current straps will no longer be compatible with newer watches and you'll have to buy the straps again.